EXPERIMENTAL ACFT WITH REGULAR CERTIFIED ACFT ENG HAD SEVERE ENG BACKFIRES IN ZNY CLASS E AIRSPACE.

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ON JUN/XA/01 AT ABOUT XA30; I DEPARTED HPN IN MY VELOCITY 173RG VFR FOR HEF VIA SPARTA VOR AND LNS. AT ABOUT 20 NM SW OF SPARTA; I DECIDED TO OUT-CLB THE SMOG AND LEFT 4500 FT FOR 8500 FT. I WAS RELEASED BY ZNY AND DSNDED WITHOUT A HDOF BEFORE I COULD ESTABLISH CONTACT WITH AN ATC FACILITY; TO OBTAIN WX DATA FROM FLT WATCH. THEN I HEARD THE FIRST OF 2 LOUD EXPLOSIONS FROM THE COWLING AT REAR OF THE PLANE. I RAPIDLY RETARDED THE THROTTLE AND BROADCAST A MAYDAY EMER. I FEARED FIRE AND POSSIBLE STRUCTURAL DAMAGE. NEITHER PROVED TRUE AND I NEVER LOST CTL OF THE ACFT. I THINK ZNY WAS THE FIRST TO RESPOND SINCE I DON'T RECALL CHANGING FREQS ON MY #2 XMITTER. I RECEIVED VECTORS TO NEAREST ARPT; ABE; 6 MI AWAY. I NEVER SMELLED SMOKE; BUT CONTINUOUS SMALLER BACKFIRING PERSISTED. AS I APCHED ABE; FLT WATCH CALLED TO VERIFY WHERE I WAS LNDG. I MUST HAVE MADE A CALL TO THEM ON MY #1 XMITTER BEFORE THE FIRST BIG BANG. BECAUSE OF THE STRESS AND RAPID PACE OF EVENTS; I CAN'T RECALL THE SEQUENCE OF SOME SPECIFICS. ABE TWR ASKED IF I NEEDED FIRE EQUIP. I RESPONDED IN THE NEGATIVE; BUT THEY CAME ANYWAY. BY THEN; I SURMISED I HAD EXPERIENCED A LOUD BACKFIRE AND NOT AN EXPLOSION. 3 FACTORS CONTRIBUTED TO MY DECISION NOT TO DELAY IN MAKING A MAYDAY CALL. 1) I HAD BEEN HAVING WORK DONE ON MY ENG FOR BACKFIRING WHILE TAXIING AND DURING SOME RUN-UPS DURING A 2 WK PERIOD PRIOR TO LEAVING MY HOME BASE; ATHENS; GA; FOR A LONG XCOUNTRY TO NEW ENGLAND. TWO SEPARATE FBO A&P'S COULD NOT FIND THE PROB. I HAD TO WAIT UNTIL IT GOT WORSE OR MORE PERSISTENT. AT THE TIME OF THE INCIDENT I WAS SURE IT HAD SUDDENLY GOTTEN MUCH WORSE. 2) MY NEW LYCOMING ENG HAS LESS THAN 500 TOTAL HRS SINCE ORIGINAL MANUFACTURE; AND NEVER BACKFIRED DURING FLT. 3) MY ANXIETY LEVEL WAS ALREADY HIGH AS I EXPERIENCED THE DETERIORATION OF VFR CONDITIONS WHILE ATTEMPTING TO GET ABOVE THE BROKEN AND SCATTERED CLOUDS. AFTER LNDG; AN A&P CLEANED AND REPOSITIONED THE O-RING IN THE #1 CYLINDER IN THE INTAKE MANIFOLD BECAUSE IT APPEARED PINCHED AND POSSIBLY COULD HAVE ALLOWED RAW AIR INTO THE SYS; CONTRIBUTING TO CONDITIONS FOR BACKFIRING. NOW AFTER 5 HRS SINCE THE ABOVE PRECAUTIONARY REPAIR; I HAVE NOT EXPERIENCED ANY FURTHER BACKFIRES. I WILL; HOWEVER; CHK INTO THIS MORE THOROUGHLY. THIS EXPERIENCE TAUGHT ME: 1) THE FOLLY OF TRYING TO OUT-CLB A SMOGGY AIR MASS IN HUMID AFTERNOON; SUMMER CONDITIONS. 2) IT'S PREFERABLE TO MAINTAIN VFR BELOW THE SMOG KEEPING GND CHKPOINTS AND ARPTS IN SIGHT AND THUS KEEPING STRESS LEVELS LOW SO THAT LOUD BACKFIRES WON'T SOUND LIKE VIOLENT EXPLOSIONS.